Coherent light scattered by reflection or transmission from a rough surface forms an interference pattern in the space away from the rough surface. On viewing the pattern, an eye sees dark and light in a granular pattern, which is the speckle. An intensity detector of an optical system will also detect the speckle, and a measure of the speckle, termed its contrast, is typically a function of the ratio of the root mean square of the intensity fluctuation to the mean intensity. The varying intensity caused by the speckle may impact unfavorably on measurements made with coherent sources such as lasers, necessitating reduction or elimination of the speckle.
U.S. Pat. No. 6,367,935 to Wang, et al., whose disclosure is incorporated herein by reference, describes a system for eliminating image speckle in a scanning laser projector. A phase hologram divides an illumination beam of the projector into partial beams. The partial beams each illuminate part of a screen area, and generate speckle patterns which are moved. The different patterns are combined so as to average the intensities, the averaging removing the speckle.
U.S. Pat. No. 6,577,429 to Kurtz, et al., whose disclosure is incorporated herein by reference, describes a laser projection display system. The system includes an electrically controllable de-speckling modulator, which reduces the speckle by providing local random phase changes for incident light in the system.
U.S. Pat. No. 6,798,505 to Karpol, et al., whose disclosure is incorporated herein by reference, describes a system for article inspection including a method for reducing speckle occurring during the inspection. An optical fiber bundle is placed in the path of a coherent light beam, and the bundle outputs multiple divergent beams having reduced coherence.
U.S. Pat. No. 6,830,189 to Tsikos, et al., whose disclosure is incorporated herein by reference, describes illumination of objects with planar laser beams. A high resolution control system modifies the phase of the wavefront of a coherent beam. The beam then produces numerous speckle patterns, which can be spatially and/or temporally averaged to reduce the observed speckle.
U.S. Pat. No. 6,169,634 to Sirat, whose disclosure is incorporated herein by reference, describes a system for converting a collimated coherent light beam into an incoherent beam. Different portions of the beam are delayed by different amounts by passage through different cells of an optical element. The different delays introduced by the optical element exceed a coherence length of the beam.